MWI and the entangled photon experiment

[kered rettop]

[Moderator's note: Thread spun off from previous discussion of the experiment due to focusing on the MWI as an interpretation, so discussion belongs in the interpretations subforum. Previous thread is linked to below.]

https://www.physicsforums.com/threa...photons-split-into-2-different-paths.1055786/

To be clear about path length: changing length will never change the quantum statistical prediction. The coincidence probability depends on the relationship between the A and B pairs, but does not depend in any way on time ordering or path length. Consequently, as @DrClaude stated, lengthening one path to make it appear that one photon is measured before the other does not change anything.

The effect you refer to as retrocausal is an artifact of trying to describe what happens as if one element of the setup is the cause of something else in the setup. To do that, you must rely on a particular interpretation of QM. Experimentally, you cannot distinguish cause from effect when asking: does measurement of A cause/change the outcome of B, or does measurement of B cause/change the outcome of A? These scenarios cannot be distinguished.

It is also an artifact of assuming that observations are definite. Under MWI, for instance, every measurement results in an indefinite outcome, with different outcomes occurring in different branches. No need for A and B to influence each other at all, let alone with retrocausality.

 

[DrChinese]

It is also an artifact of assuming that observations are definite. Under MWI, for instance, every measurement results in an indefinite outcome, with different outcomes occurring in different branches. No need for A and B to influence each other at all, let alone with retrocausality.

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.*Local versions, of course. Some see MWI as non-local.

 

[kered rettop]

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

I was saying that the global outcome is a superposition so it's not definite.

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

MWI does predict the existence of such worlds. But they are very rare so their frequency is vanishingly small.

*Local versions, of course. Some see MWI as non-local.

Well interactions are certainly local in MWI. The non-locality appears to be what I think you referred to as "contextual" recently - for example, Alice and Bob's settings. The debate would therefore hang on how world-splitting depends on the (quantum) context. I don't know whether your discussion was ever resolved, but I was very glad to see it as it has considerably dampened my enthusiasm for MWI.

 

[PeterDonis]

There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

This is not correct. We already went over this in great detail in another thread. The MWI does not say this and does not work this way. We ended up agreeing that those who claim that the MWI is "local" are not correct, but we did not end up agreeing that the MWI cannot explain the correlations. Its explanation is not "local", but that doesn't mean it isn't valid.

 

[PeterDonis]

MWI does predict the existence of such worlds.

Only if the alignment between the distant measuring devices is not perfect. In the idealized case where the alignment is perfect, the MWI predicts that there are no worlds in which the perfect correlation is not present. This was discussed in great detail in another thread.

 

[PeterDonis]

interactions are certainly local in MWI.

Yes, but the wave function is not; it explicitly includes entangled degrees of freedom that are spatially separated, and a measurement on one of those degrees of freedom instantaneously updates the entangled wave function, which affects all of the entangled degrees of freedom. This was discussed in great detail in another thread.

 

[PeterDonis]

The previous thread I referred to is this one:

https://www.physicsforums.com/threads/is-the-mwi-local.1057817/

 

[kered rettop]

Only if the alignment between the distant measuring devices is not perfect. In the idealized case where the alignment is perfect, the MWI predicts that there are no worlds in which the perfect correlation is not present. This was discussed in great detail in another thread.

OK. I'd overlooked the perfect alignment. But in that case, I can't make any sense of what DrChinese says next: "Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly." How can an outcome occur at all if it has precisely zero amplitude?

 

[kered rettop]

The previous thread I referred to is this one:

https://www.physicsforums.com/threads/is-the-mwi-local.1057817/

Great, that's what I was looking for. Thanks.

 

[PeterDonis]

in that case, I can't make any sense of what DrChinese says next

Yes, that's why I referred to the previous thread.

 

[PeterDonis]

all outcomes occur randomly

I'm commenting on this separately because I can't remember how much it was discussed in the previous thread. This statement is wrong independently of the other issues I referred to before: there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

 

[DrChinese]

This is not correct. We already went over this in great detail in another thread. The MWI does not say this and does not work this way. We ended up agreeing that those who claim that the MWI is "local" are not correct, but we did not end up agreeing that the MWI cannot explain the correlations. Its explanation is not "local", but that doesn't mean it isn't valid.

I had a * beside MWI specifically to avoid this comment from you. I failed, obviously...

 

[kered rettop]

Yes, that's why I referred to the previous thread.

<irony>Well, I'll have a good look at it then, and let you know who's right. </irony>

 

[DrChinese]

...there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

You say tomato, I say tomato. What branch we exist in now is purely random, there really is no denying that point. So what if all branches allegedly occur? I can't begin to call that deterministic. And I say there is no way to wind any quantum system evolution backward to the past or forward to the future from the branch we are in now. Apparently, "something" discontinuous happens to split worlds, whether you call it "collapse" or something else. You obviously cannot see that in the past of a particle, nor can you see it in the future of any particle. How many papers* have been written explaining that it is meaningless to discuss what happens in the quantum world when no one is looking?

There are articles such as Vaidman's 2014 (a great paper, by the way, regardless of your viewpoint) that argue (exactly as you say) that MWI is both local and deterministic, and there is no randomness. I don't find it convincing, but it is almost an encyclopedia of Interpretations and No-Go's.*Such as Vaidman 2013 (different paper than above) who also said this, for example: "Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer ...we conclude that the past of the photons is not represented by continuous trajectories, although a “common sense” analysis adopted in various welcher weg measurements, delayed-choice which-path experiments and counterfactual communication demonstrations yields a single trajectory."

 

[PeterDonis]

I had a * beside MWI

Which means what? The MWI is what it is. There is no asterisked version of it that works differently.

 

[DrChinese]

<irony>Well, I'll have a good look at it then, and let you know who's right. </irony>

Read the Vaidman 2014 paper also. Although I disagree strongly with his final conclusion(s)*, he covers the pros and cons better than anything I've seen anywhere else. It's 25 pages, and 175 references!

*"The theory of Universal wave function is deterministic, local, free of paradoxes, and fully consistent with
our experience."

 

[PeterDonis]

What branch we exist in now is purely random

Wrong. "We" exist in all branches.

There may be other interpretations that say something along the lines of "all of the branches exist mathematically, but which one becomes real and we end up in is randomly chosen". But the MWI does not say that. If you want to use one of those other interpretations instead of the MWI, that's fine, but then please correctly describe which interpretation you are actually using.

there really is no denying that point.

Yes, there is. You are making repeated wrong claims about what the MWI says.

So what if all branches allegedly occur?

There is no "allegedly". They all occur, because they are all in the wave function.

I can't begin to call that deterministic.

In the MWI, the wave function's time evolution is unitary, always, all the time. Unitary evolution is deterministic. That is just a mathematical fact. If you are using an interpretation that does not say this, whatever it is, it is not the MWI.

There are articles such as Vaidman's 2014 (a great paper, by the way, regardless of your viewpoint) that argue (exactly as you say) that MWI is both local and deterministic, and there is no randomness.

You misdescribe the paper. Vaidman is not arguing about what the MWI is; he is just stating what the MWI is. The only caveat he does not mention is that his usage of the term "local" is limited: it does not mean the wave function is local (it's not), it just means that individual interactions, which only operate on degrees of freedom that are spatially co-located, are local.

 

[PeterDonis]

deterministic, and there is no randomness

And these two things are obvious, simple consequences of the basic premises of the MWI: that the wave function is real and contains all of reality, and that the wave function's time evolution is unitary, all the time. While there are different presentations and formulations of the MWI in the literature, and different proponents might disagree about some particular fine points, all of them agree on those two premises as the fundamental basis of the MWI.

 

[DrChinese]

Which means what? The MWI is what it is. There is no asterisked version of it that works differently.

I don't think MWI can be considered local (the asterisk was to denote claims of locality in MWI). For it to make any sense, IMHO, there must be nonlocality of some kind. Anyone can claim anything about an interpretation, but at some point it needs to be reconciled with experiment. If someone says it is local, there is the issue of how to explain the thousands of experiments on nonlocality - entanglement swapping for starters.

I have referenced Vaidman's strong defense of MWI as local and deterministic with a comprehensive paper he wrote. I will quote the entirety of his discussion on entanglement swapping (actually quantum teleportation):

"Spatially separated entangled particles, through local interaction with macroscopic objects, create worlds with nonlocal correlations. Measurement of one particle of the EPR pair changes nothing for the other particle if it is considered in the Universe, but it creates worlds with definite spin of a remote particle.
"If the EPR pair is used for teleportation, the Bell measurement in one site creates four worlds with the quantum state teleported to the second particle of the pair and rotated in four definite ways. The mixture of these four states corresponds to a completely unpolarized density matrix, the description of the particle of an undisturbed EPR pair. Thus again, from the point of view of the Universe, no change in the second EPR particle took place. "

If that's not hand-waving, I don't know what is. We have previously discussed that an experimenter can freely choose to entangle (i.e. nonlocally change the state of) remote systems. Vaidman apparently acknowledges this, saying: "Entanglement is the essence of the nonlocality of the Universe." So he says this, and then concludes MWI is local at the end of the paper.

My point is that MWI makes some sense as an interpretation when you consider the idea that there is a Universal wave function, but... there must still be nonlocal elements. Just saying its "local" isn't enough in today's world. We've come too far for that.

 

[kered rettop]

I'm commenting on this separately because I can't remember how much it was discussed in the previous thread. This statement is wrong independently of the other issues I referred to before: there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

Making sense of probability - I still don't understand the problem. It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it. Is it possible to discuss the matter without incurring the wrath of the mods?

 

[Nugatory]

all outcomes deterministically occur.

Agree about the determinism, but I would be more comfortable saying something along the lines of “all outcomes are deterministically in the wave function“ instead because that’s as far as the math (deterministic evolution of the state) will take us. "Occurs" carries much more ontological baggage - there's a big gap between a wave function with post-decoherence amplitude peaks for each outcome and what has occurred in the operational sense of the word.

 

[PeterDonis]

I don't think MWI can be considered local (the asterisk was to denote claims of locality in MWI).

Ok. Yes, we agreed in the previous thread that neither of us could see how the MWI could be completely local. Interactions are, but the wave function is not.

 

[PeterDonis]

It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it.

If there is such a simple solution, it should already be in the literature. People have been discussing this topic for decades. There is plenty of literature on how to make sense of probabilities in the MWI. Please take the time to work through it.

 

[PeterDonis]

I would be more comfortable saying something along the lines of “all outcomes are deterministically in the wave function“ instead because that’s as far as the math (deterministic evolution of the state) will take us. "Occurs" carries much more ontological baggage

Sure, but the baggage is the same whether you say that all outcomes occur (which is the MWI) or that only one occurs and which one it is is random (other interpretations that are not the MWI). So I don't see this as a reason not to say "all outcomes occur" in the MWI. You have the same problem saying that just one outcome occurs in, say, the Copenhagen interpretation, and nobody shies away from saying that an outcome occurs in the Copenhagen interpretation and insists on saying "there is an outcome in the wave function" instead.

 

[DrChinese]

1. "We" exist in all branches.

2. You mis-describe the paper. Vaidman is not arguing about what the MWI is; he is just stating what the MWI is. The only caveat he does not mention is that his usage of the term "local" is limited: it does not mean the wave function is local (it's not), ...

3. ...it just means that individual interactions, which only operate on degrees of freedom that are spatially co-located, are local.

1. That's the hypothesis.

But there are no other "we's" to confirm that hypothesis, nor is there the slightest evidence to indicate anything other than we live in a random branch - if there are other branches.

Every single experiment, every day of every week, says the outcomes of all quantum experiments are random. There is no experimental evidence to the contrary. To describe that as "deterministic" is a gross mischaracterization of the word. Even considering that the universal wave function evolves deterministically, I don't see how you describe the quantum world as deterministic. Obviously, there is supposed to be branching - and how can that be called pre-determined?2. Of course he is arguing for MWI. And I think it is a pretty good defense at that. He deftly compares it to some of the most important alternative ideas, and he goes through a pretty long list of quantum phenomena. Very comprehensive, and fairly up to date.3. I'm not sure I agree that "individual interactions, which only operate on degrees of freedom that are spatially co-located, are local." I mean, I guess that's technically true if you parse it a certain way. But there is certainly no strict requirement that individual quantum interactions only operate on properties (degrees of freedom) that are spatially co-located. Elements of a general quantum interaction need no specific locale, and may or may not be co-located.

One of Vaidman's many specialties is systems where there are quantum interactions that are NOT spatially co-located. That's indicated in his prolific work with Mach-Zehnder systems, specifically his work on weak measurements - and of course there's his bomb tester.

I certainly wouldn't call an individual interaction "here" (such as Bell State Measurement) that changes a system far distant elsewhere "local" anyway. Where would you even begin to localize such interaction? Let's start with the Bell State Measurement apparatus itself. A Bell State Measurement occurs in a system of 3 optical splitters and 4 detectors (let's ignore the experimenter for now). Those 7 components can be located anywhere, miles apart even. So when and where is there a "local interaction"? Yet again, the relevant quantum context is completely nonlocal. None of the component events need to occur simultaneously (except for a small area in one splitter where there is overlap to induce indistinguishability). But the selection of the Bell State (1 of the possible 4) is done by the remaining components - which can be done anywhere at any relative time.

---

@PeterDonis : I don't feel our back and forth is helping those who might otherwise be interested in this thread. So I will note your objections to my descriptions of MWI, and try to stay away from further comments that I know you will disagree with.

Apologies offered to the extent I may have led the thread astray. I will again encourage anyone reading this to look at the reference on Vaidman's MWI work.

 

[kered rettop]

If there is such a simple solution, it should already be in the literature. People have been discussing this topic for decades. There is plenty of literature on how to make sense of probabilities in the MWI. Please take the time to work through it.

I'll take that as a "no", then.

 

[PeterDonis]

That's the hypothesis.

It's part of the interpretation. Calling it a "hypothesis" seems odd, because you can't test interpretations; they all make the same predictions for experimental results. They're interpretations. There is no way to test an interpretation experimentally. You can be skeptical about an interpretation because of claims it makes that seem extravagant to you--for example, the MWI claims that all branches of the wave function really exist even though we, in whatever branch we are in, have no way of ever testing whether that claim is true.

Of course he is arguing for MWI.

In the sense that he appears to be a proponent of it, yes. But his descriptions of what it says are not arguments, as if he needed to prove or establish somehow that that's what the MWI says. He's just describing what it says. And then he makes arguments for why he favors it as an intepretation.

One of Vaidman's many specialties is systems where there are quantum interactions that are NOT spatially co-located.

I would not describe those scenarios that way. I would describe them as scenarios in which quantum nonlocality is brought out: a local interaction on one degree of freedom affects some other non-co-located degree of freedom. Or, in some cases (like the bomb tester), the lack of an interaction on one degree of freedom affects another non-co-located degree of freedom.

To an extent these are just debates about words, but if we're going to look at particular references we should try to be clear about how they use words, even if we ourselves might not agree with their choices of words.

 

[PeterDonis]

Every single experiment, every day of every week, says the outcomes of all quantum experiments are random. There is no experimental evidence to the contrary. To describe that as "deterministic" is a gross mischaracterization of the word.

I'm sorry, but all this is completely irrelevant to the actual point I made. I gave a perfectly valid reason for why the MWI is deterministic. Nothing you say makes that reason invalid. You are not saying anything that actually rules out the MWI as an interpretation. Nor are you saying anything that changes what the MWI says as an interpretation. And that is what my posts are about: what the MWI says as an interpretation. I am not trying to argue that the MWI is true. I am only trying to correct your mistaken claims about what it says.

 

[PeterDonis]

I don't feel our back and forth is helping those who might otherwise be interested in this thread. So I will note your objections to my descriptions of MWI, and try to stay away from further comments that I know you will disagree with.

Please bear in mind that, as I said in my previous post just now, I am not trying to argue that the MWI is true. I don't actually think it is; I am not an MWI proponent as a matter of personal opinion. I am only trying to make sure it is clear what the MWI says. That is the only reason I have objected to your posts: that they are incorrect as descriptions of what the MWI says. I am not at all claiming that your posts are incorrect as a description of how things actually are, or as a description of what we actually observe in experiments. Those are separate questions from the question of what the MWI, or any interpretation, says.

 

[PeterDonis]

I don't feel our back and forth is helping those who might otherwise be interested in this thread...

Apologies offered to the extent I may have led the thread astray.

I have addressed this by spinning this discussion off into a new thread in the interpretations subforum.

 

[gentzen]

How many papers* have been written explaining that it is meaningless to discuss what happens in the quantum world when no one is looking?
...
*Such as Vaidman 2013 (different paper than above) who also said this, for example: "Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer ...we conclude that the past of the photons is not represented by continuous trajectories, although a “common sense” analysis adopted in various welcher weg measurements, delayed-choice which-path experiments and counterfactual communication demonstrations yields a single trajectory."

Looks like this reference is more about the two-state vector formalism and the related theory of weak values than about it being "meaningless to discuss what happens in the quantum world when no one is looking". What is true is that the past of the photons is not represented by continuous trajectories of point particles for the experiments described in the paper. Instead, there is that "two state vector" object with better spatial localization and a better "claim for existence" than the wavefunction (which only exists in the form of probabilies for outcomes of potential measurements). The point of the weak values is that you can look without disturbing things too much, and hence it is meaningful to discuss them even when no one is looking.

(I guess you found that refence while collecting TSVF references, but then decided to include it here instead, because even so it is an extremely nice paper, it didn't follow the pattern of your other references. Namely, it didn't include Aharonov as coauthor, and gave measurement data from "real" experiments instead of focusing on theory.)

 

[kith]

Sure, but the baggage is the same whether you say that all outcomes occur (which is the MWI) or that only one occurs and which one it is is random (other interpretations that are not the MWI). So I don't see this as a reason not to say "all outcomes occur" in the MWI.

I don't disagree but I want to highlight that it isn't the most natural thing to say from a MWI perspective.

When we perform a measurement, our fundamental notion from a Copenhagen perspective is that a single outcome occurs. Our fundamental notion from a MWI perspective is that a branching of worlds occurs. This leads to multiple instances of ourself experienceing different single outcomes.

I wonder if both @PeterDonis and @DrChinese (or neither of them ;-)) would agree with the MWI saying "the worlds of all instances are equally real" and "every instance experiences only a single outcome".

 

[vanhees71]

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

That's precisely what I don't understand about the MWI interpretation. On the one hand they say there is this branching, but on the other we have definite outcomes when we are doing the experiments. So what does this branching then mean from an experimental/observational point of view?

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

I don't think so, because the perfect correlations are properties of the entangled state. E.g., if you have a polarization-singlet state of two photons, when finding photon 1 H-polarized then the other photon will be found V-polarized with probability 1 and vice versa. Which of these to possible outcomes of the measurements on the two photons is of course completely random, but the 100% correlation is always (with 100% probability) occuring. That means according to MWI there are two branches when making this measurement: photon 1 H and photon 2 V and another with photon 1 V and photon 2 H polarized.

 

[DrChinese]

I don't disagree but I want to highlight that it isn't the most natural thing to say from a MWI perspective.

...

I wonder if both @PeterDonis and @DrChinese (or neither of them ;-)) would agree with the MWI saying "the worlds of all instances are equally real" and "every instance experiences only a single outcome".

First, thanks to @PeterDonis for splitting this off the main thread.

Sure. I'd agree with that @kith. And I certainly am not trying to mischaracterize MWI by my choice of words. I will attempt to use the descriptive phrases that are more in keeping with the usual MWI terms, even if I don't think they really apply that well.

I am going to reply to @vanhees71 's post #33 to discuss some of the issues. My reply should not be considered any disagreement with him.

 

[gentzen]

Making sense of probability - I still don't understand the problem. It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it. Is it possible to discuss the matter without incurring the wrath of the mods?

Starting an answer in an existing thread with "I still don't understand ..." risks to annoy the other participants in that thread. If there is something which you don't understand, why don't you ask that as a separate question in a new thread? On the other hand, if you really want to give an answer in an existing thread, then requesting you to find a reputable reference in case your answer causes confusion or disagreement seems reasonable.

However, sometimes even slight hints that somebody might be trying to promote personal research seem to get him banned. Jarek Duda is certainly not a crank, and his research is often creative, novel and impactful. I guess he had received warnings before about which parts of his behavior won't be tolerated. And I guess that quoting poor references was not part of it.

That said, I would be curious about your simple solution. But not in this or any other existing thread. I also would prefer if you do some research for references first (and tell us if you found nothing related), so that I as reader am at least spared the effort to research whether your solution is completely new, or not.